A dispersion liquid blending metallic copper particles is a general term that usually includes compositions, such as coating agents, paints, pastes, and inks, which is obtained by dispersing metallic copper particles in solvent, and further blending therein an additive such as a binder, a dispersant, and a viscosity modifier when necessary. Such a dispersion liquid is used, by taking advantage of the characteristics of the metallic copper particles, in various applications such as applications to secure electrical conduction, applications to prevent electrification, applications to shield electromagnetic waves, applications to give metallic luster or antibacterial properties. Additionally, metallic copper particles having an average particle diameter of about 1 to 200 nm have been used in recent years as the metallic copper particles which are blended, and its applications have been expanding to various fields. Specifically, the metallic copper particles are used, by taking advantage of a high electrical conductivity thereof, for electromagnetic wave shielding of transparent members such as a cathode-ray tube and a liquid crystal display. Moreover, technique of forming a fine electrode or a fine circuit-wiring pattern by the use of the metallic copper particles of nanomaterials has been proposed. This technique is as follows: the dispersion liquid blending the metallic copper particles coats a substrate to form an electrode pattern or circuit-wiring pattern by a coating method such as a screen printing or inkjet printing, and thereafter the metallic copper particles are fused by heating at a relatively low temperature. This technique has been applied particularly to production of printed wiring boards. Furthermore, the fusion of the metallic copper particles easily progresses to exhibit metallic luster even under the conditions of a mild heating, and thus the dispersion liquid blending the metallic copper particles has been attracting attention in design and decoration applications as a simple technique for producing a mirror surface.
Regarding metallic copper particles or a dispersion liquid dispersing the metallic copper particles, Patent Literature 1 for example discloses that the metallic copper particles are obtained by mixing a divalent copper oxide and a reducing agent in a liquid medium in the presence of a complexing agent and a protective colloid to reduce the divalent copper oxide, and that the obtained metallic copper particles are dispersed in a dispersion medium to make a fluid composition, and also discloses that a water solvent, hydrophilic organic solvents, hydrophobic organic solvents, and the like are used as the dispersion medium. Moreover, Patent Literature 2 describes a dispersion liquid containing the metallic copper particles having gelatin on the surfaces of the particles, a polymeric dispersant, and an organic solvent, wherein the gelatin has a difference between an amine value and an acid value (amine value−acid value) of 0 or less and the polymeric dispersant has a difference between an amine value and an acid value (amine value−acid value) of 0 to 50.